Frequency and phase controllers, such as disciplined oscillators as described in the above-referenced U.S. patent and also the Stratum 2 disciplined oscillator sold by Spectracom Corporation, East Rochester, New York 14445 U.S., have used digital techniques for locking a standard oscillator to a reference and even removing long term phase and frequency variations to high degrees of precision. In such systems the reference frequency source is reliable (essentially uncorrupted by noise and other errors) for long periods of time, or the reference frequency source is located close by the controlled standard source and provides continuous, essentially noise and error free reference signals. The reference signals are also available at high frequencies, for example 10 Mhz so that frequency and phase errors are detected quickly while the reference is monitored.
In such applications as simulcast transmission systems where precise frequency control is required so as to prevent interference in zones where transmissions from different transmitters overlap, and in other applications where frequency and/or phase control of the local standard signal source is required, a local reference signal is not available. It is desirable that the signal be transmitted to sites remote from the reference source. This, however, adds noise and other errors which cause the phase of the reference to jitter and the reference to vary in amplitude. Also, there may not be a separate link for reference signals over which the reference frequency can be transmitted at high frequency. It is desirable to place the reference frequency within the band width of the other signals which are being communicated for transmission. In simulcast transmission these signals occupy the voice band width (300 to 3,000 Hz). The problem is exacerbated in simulcast transmission since customer traffic (pages in paging simulcast systems) must be given preference to calibration of the transmitters to the precise carrier frequencies required for proper system performance. Thus, it is desirable that the frequency calibration be carried out using information which is available only intermittently, perhaps at times separated by hours, and yet maintain the requisite precise frequency control of all of the transmitters.
In simulcast transmission systems frequency control is conventionally accomplished by technicians who must visit the remote transmitter sites and align the transmitters in frequency. Alignment of the transmitter's modulation characteristics (delay, gain, manually at the transmitter sites. If the modulation characteristics of the transmitters are not aligned, interference arises particularly in zones where the transmissions overlap. It is desirable that such alignment be carried out automatically, on a regular schedule or at times when the traffic carried by the system is light.